A lighting lamp such as a fluorescent lamp, etc., has a definite lamp life and since the lifeless lighting lamp must be renewed, it has been desired to detect the lamp life left of a lighting lamp before the termination of the life of the lighting lamp for renewing the lamp. Hitherto, the lamp life of a lighting lamp is determined by observing the blackened phenomenon at the end portion of the bulb.
Also, it is known to apply an organic material such as a coating material, etc., capable of being discolored or faded by the action of ultraviolet rays to the surface of a bulb of a lighting lamp and determine the lamp life by the extent of the discoloration or fading. The display of the integrated lighting time or the previous notice of life of a lighting lamp in these conventional techniques is performed by a material capable of coloring, fading, or discoloring by the active light from the lamp or a display sheet using the material as disclosed, e.g., in JP-A-U-53-14057 and JP-A-U-622-1133366 (the term "JP-A-U" as used herein means an "unexamined published Japanese utility model application") and JP-A-64-65766 (the term "JP-A" as used herein means an "unexamined published Japanese patent application").
Now, in a display sheet, a supply of oxygen is required for causing coloring, fading, or discoloring of the active layer thereof and if the supply of oxygen is insufficient, there occurs a problem that the progress of coloring, fading, or discoloring of the active layer becomes unreliable by causing unevenness in coloring, fading, or discoloring, etc., whereby accurate display becomes impossible.
In the aforesaid conventional techniques, no consideration is made of the manner of attaching the display sheet to a lamp. There is a problem that when the display sheet is attached to a lamp, unevenness in coloring, fading, or discoloring of the active layer occurs, whereby accurate display of the life of the lamp becomes impossible.